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4.3 How Atoms Differ

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Atomic Number - Atomic Number = # protons in an atom - # of protons determines kind of atom (atoms of an element always have the SAME number of protons) - The atomic number = the number of electrons in a neutral atom - Written just above the atomic symbol on the periodic table

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Complete the Table: ElementAtomic ## Protons# Electrons 6 N Pb 3 19 Br

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ElementAtomic ## Protons# Electrons C666 N777 Pb82 Li333 K19 Br35

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Isotopes All atoms in an element have the same number of protons The number of neutrons vary Atoms of an element with different numbers of neutrons = ISOTOPES

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Isotopes Elements in nature = a mix of isotopes The abundances of different isotopes are constant EX: in ANY banana, 93% K has 20 neutrons, 7% have 22 neutrons http://www.youtube.com/watch?v=Jdtt3LsodAQ&feature=related

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Mass of atoms One atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom.atomic mass unit One amu is nearly, but not exactly, equal to one proton or one neutron.

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Mass of subatomic particles

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Mass Number # PROTONS + # NEUTRONS = MASS NUMBER EXAMPLE: What is the mass number for manganese if there are 30 neutrons in the nucleus?

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Element Symbols Top is mass number Top is mass number Bottom is atomic number Bottom is atomic number Practice Practice

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Symbols EXAMPLE 1: Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass Number F 19 9

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Answer… - - 9 protons - - 10 neutrons - - 9 electrons - - Atomic # = 9 - - Mass # = 19 - - What would the mass number be of a fluorine isotope with two additional neutrons?

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Symbols EXAMPLE: Find the –number –number of protons of neutrons of electrons –Atomic –Atomic number –Mass –Mass Number Br 80 35

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Answer –Number of protons: 35 –Number of neutrons: 45 –Number of electrons: 35 –Atomic number: 35 –Mass Number: 80

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Symbols If an element has an atomic number of 34, a mass number of 78, find… –number of protons –number of neutrons –number of electrons And WRITE the Complete symbol

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Answer… - - 34 protons - - 34 electrons - - 44 neutrons - - Symbol: Se 78 34

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Symbols If an element has 91 protons and 140 neutrons what is the……..? –Atomic number –Mass number –number of electrons –Complete symbol

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Pa 91 231

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Symbols If an element has 78 electrons and 117 neutrons what is the ………? –Atomic number –Mass number –number of protons –Complete symbol

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Pt 78 195

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Naming Isotopes Put the mass number after the name of the element Put the mass number after the name of the element carbon- 12 carbon- 12 carbon -14 carbon -14 uranium-235 uranium-235

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Mass of the atom Atomic mass on the periodic table is NOT an even # The atomic mass of an element is the weighted average mass of the isotopes of that elementatomic mass Depends on abundance of each isotope

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Atomic Mass - How heavy is an atom of oxygen? - The MASS NUMBERS we’ve been using are measured in amu - 1 amu (atomic mass units) is ABOUT the mass of 1 proton or 1 neutron (or 1/12 of a carbon -12 atom) - On the periodic table, the MASS of each element is NOT reported as a whole number… - We can’t have HALF a proton or HALF a neutron in an atom… so why aren’t these masses whole numbers??

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Atomic Mass - - Chlorine-35 is MORE common in nature than chlorine-37…so the mass of 1000 Cl atoms will be CLOSER to 35000 amu - - To take the average of the two isotopes, we need to use the WEIGHTED AVERAGE: - - In nature, 75.53% of Cl atoms are chlorine-35, and 24.47 % are chlorine- 37… - - (% is called % abundance)

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Calculating the WEIGHTED AVERAGE - - Write each percent as a decimal - - Multiply that decimal by the isotope’s mass - - Find the average by adding together Average = % as decimal x mass + % as decimal x mass - Average Mass of a Chlorine atom: (0.7553)(35) + (0.2447)(37) = 35.4894 amu (0.7553)(35) + (0.2447)(37) = 35.4894 amu % abundance isotope 1 Mass isotope 1 % abundance isotope 2 Mass isotope 2

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Atomic Mass: EXAMPLE - Calculate the atomic mass of copper if copper has two isotopes. 69.1% have a mass of 62.93 amu and the rest have a mass of 64.93 amu…

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ANSWER… Mass of Cu = (.691)(62.93) + (.309)(64.93) = 63.548 amu

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Atomic Mass: EXAMPLE 2 - Magnesium has three isotopes. 78.99% magnesium-24, 10.00% magnesium-25, and the rest is magnesium-26 amu. What is the atomic mass of magnesium?

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Answer… Mass of Magnesium = 7899 x 24) + (.1000 x 25) + (.1101 x 26) = (. 7899 x 24) + (.1000 x 25) + (.1101 x 26) = 24.3202 amu

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